docs: def clean_username(self):
          """
          Validate that the username is alphanumeric and is not already in use.
          Also validates that the username is not listed in
          ``USERENA_FORBIDDEN_USERNAMES`` list.
  
          """
          try:
              user = get_user_model().objects.get(username__iexact=self.cleaned_data['username'])
          except get_user_model().DoesNotExist:
              pass
          else:
              if userena_settings.USERENA_ACTIVATION_REQUIRED and UserenaSignup.objects.filter(user__username__iexact=self.cleaned_data['username']).exclude(activation_key=userena_settings.USERENA_ACTIVATED):
                  raise forms.ValidationError(_('This username is already taken but not confirmed. Please check your email for verification steps.'))
              raise forms.ValidationError(_('This username is already taken.'))
          if self.cleaned_data['username'].lower() in userena_settings.USERENA_FORBIDDEN_USERNAMES:
              raise forms.ValidationError(_('This username is not allowed.'))
          return self.cleaned_data['username']